The plight of those framed in this way is all the worse because paedophiles commonly use supposed malware infections of their PCs to explain the presence of images of child abuse. Because of this the "Trojan did it" defence is understandably met with scepticism from law enforcement professionals.

AP interviewed former child abuse suspects who were arrested after depraved images were found on their computers, as well as police and computer forensic experts.

Michael Fiola, a former Massachusetts government worker, was arrested two years ago after child abuse images were discovered on his state-issued laptop computer after officials became suspicious of huge data use bills associated with the machine and began an investigation. He was eventually cleared nearly 11 months later after defence experts were able to show that the laptop harboured malware programmed to visit as many as 40 child porn sites per minute, far faster than a human surfer would be able to accomplish.

Forensics experts hired by the prosecution agreed with these findings and the case, which had nearly ruined Fiola's life, was dropped.

Fiola was fired from his job before enduring death threats and losing friends. His wife stood by him, however, and the couple were able to raise a $250,000 legal defence fund after selling their car, cashing in their savings and re-mortgaging their home. "It ruined my life, my wife's life and my family's life," Fiola told AP.

A cap on the amount of damages they might receive has effectively prevented the Fiolas from suing the state.

Child abuse webmasters sometimes use either compromised consumer or business systems as a warehouse for child abuse images and videos. Paedophile images can also land on systems as the result of a nasty prank or as an act of deliberate sabotage.

Two UK men were cleared of child-abuse offences in 2003 after it was shown that computer viruses were behind the presence of child abuse images on their PCs. One of the PCs was infected by a virus that changed the home page of the Windows machine to a child porn site. The other man's machines was infected by a virus that downloaded images of child abuse.

One of the two unnamed men lost custody of his seven-year-old daughter and spent a week in jail on remand because of the case.

In another case Chris Watts, a British computer forensics expert, helped clear a hotel manager whose colleagues found child porn on a shared workplace PC. The manager had been looking for ways to download pirated computer games when his web session was redirected to a child abuse site.

More details on the cases and discussion of the issue can be found here. ®